In a hydraulic circuit in a press brake according to the prior art, an upper cylinder chamber and a lower cylinder chamber in a hydraulic cylinder are respectively connected to a switch valve via piping. The hydraulic cylinder moves a ram. For example, an upper table or the like are respectively connected to a switch valve via piping. However, since an opening degree of the switch valve and a supply flow amount (to the upper cylinder chamber and the lower cylinder chamber in the hydraulic cylinder) are not in a linear relationship, it is impossible to monitor a speed on the basis of the opening degree of the switch valve. Accordingly, the ram speed is monitored only by a ram sensor. Therefore, there has been a problem that the ram speed cannot be corrected if the ram sensor is out of order.
This invention has been made with consideration to the problem in the prior art mentioned above, and an object of this invention is to provide a safer ram speed monitoring method and a press brake which uses the ram speed monitoring method.
Another object of this invention is to provide a ram position control method and a ram position controller which can position a ram at a position with no relationship to a pressure and a temperature of a working fluid supplied from a hydraulic pump to a hydraulic cylinder.